Our long-term goal is to understand the molecular details and mechanisms involved in protein synthesis. Towards this goal, we will investigate the termination of protein synthesis in this proposal. The termination of protein synthesis includes accurate decoding of the stop codon by the class I release factors, hydrolysis of the ester bond between the nascent peptide chain and P-site tRNA in the ribosome, and the resulting release of the polypeptide from the ribosome. A detailed understanding of this process requires high- resolution structures of the relevant functional complexes. These structures will reveal molecular details of the interactions that occur during translational termination between the ribosome, the P-site tRNA, the stop codon and release factors. I propose to determine high-resolution crystal structures of various functional complexes along the translational termination pathway. These structures will be exploited to direct mutagenesis and biochemical studies, leading to insights into the mechanisms of the binding of the release factors to the ribosome, the specificity of stop-codon recognition and the catalysis of peptide release. Specific aims: (1). To determine high-resolution crystal structures of the release complexes with different factor-codon combinations. (2). To determine high-resolution crystal structures of the release factors bound to ribosome with nonhydrolyzable tRNA substrate analogs at the P-site of the ribosome that emulate the translational state directly before peptide hydrolysis. (3). To study the mechanistic details of the release factor mediated stop-codon recognition and peptide release using genetic and biochemical methods. PUBLIC HEALTH RELEVANCE: Translation is a fundamental process in all cells. Many viruses hijack the translation machinery by circumventing the normal translation pathway, and deregulation of translation is involved in various cancers. Detailed mechanistic studies of translation will be essential to understand these phenomena.